Optical communications are known in which optical signals are modulated in accordance with various Quadrature Amplitude Modulation (QAM) formats, in which the amplitude and phase of the optical signals are modulated to carry data. In accordance with such QAM modulation formats, multi-bit sequences or symbols are represented as a particular combination of phase and amplitude of an optical signal at a given instant in time. The symbols may be represented as a constellation of points in a so-called “IQ plane.” 8QAM may refer to a known class of QAM formats in which three bit sequences or symbols are carried by an optical signal. The symbols may be represented by a eight points arranged in a variety of constellations. One such constellation is a so-called star-8QAM constellation in which four inner point that are closer to the origin of the IQ plan are surrounded by four outer points, such that each outer point is equidistant to the two inner points that are closest to it.
In the star-8QAM constellation, however, an erroneous transition, instead of to the intended outer point, is equally likely to either one of the nearest inner points. Typically, the symbol associated with one of the inner points has two bits that differ from the non-errored outer point. Such “two-bit errors” increase the bit error rate (BER) of optical signals which have been modulated in accordance with a star-8 QAM constellation.